The invention relates to a load-independent control device for hydraulic users.
Described in German DE 25 14 624-A1 is a load-independent control device for hydraulic devices, comprising a control device for the hydraulic device and a directional control valve that can be controlled independently of the control unit. The directional valve has a through-position in which it conducts the load pressure to the reservoir, and a blocking position in which the load pressure is blocked from the reservoir. The control unit is brought from its neutral position into one of its operating positions by means of an external, separate actuator. At least one hydraulic device is connected to a pump via service lines. Through the pressure that then builds up in a sensor line, the directional control valve is moved into a blocking position in which it blocks the passage between a discharge pipe and a reflux line. Pressure control in the service lines takes place through a pressure regulating valve arranged upstream from the control unit.
Since the directional control valve can only occupy its blocking position when the control unit and the pressure regulating valve have assumed the volumetric flow regulating function, this results in a relatively sluggish response of the control device. This can have consequential effects particularly when a hydraulic system equipped with a control device of the type described above is switched from so-called stand-by mode to so-called load-sensing mode, in which a hydraulic quantity that is optimally adapted to the load of the work aggregate must be made available in a very short time, because an undersupply of the hydraulic aggregates can occur in the initial phase of volumetric flow regulation.
To counteract these disadvantages, there has been disclosed in the above-referenced U.S. Pat. No 4,967,554, a control device which ensures that the above-mentioned switching of the hydraulic system from a stand-by mode to a load-sensing mode is completed before the volumetric flow regulating function for the hydraulic consuming device is performed. As a result of the hydraulic operation of the directional control valve and the control unit (formed as a servo valve), it is possible to operate both elements of the control device with little expenditure and to ensure at the same time that the pump reliably provides a sufficiently high pressure when assuming volumetric flow regulation, even if the pump was previously operated at a very low energy level, for example in a stand-by mode.
The present invention aims to improve such a system as well as to adapt such a load-independent control device to hydraulic systems equipped with a fixed displacement pump, i.e., a pump with fixed stroke volume. In such volumetric flow regulated hydraulic systems with a fixed displacement pump, as low a pump circulation pressure as possible is strived for to reduce power losses in the neutral position of the hydraulic device and also when the load-sensing line is exhausted. If an internal control oil feed is provided, it can be assumed that the desired pump circulation pressure is clearly below the required control pressure.
The invention is, therefore, based on the task of developing a load-independent control device for hydraulic devices in such a way that it also can be used, with as little switching effort as possible, in a hydraulic system having a fixed-displacement pump, the pump circulation pressure of which is below the control pressure in the neutral position of the hydraulic device.